Conventionally, there have been air conditioning systems that are equipped with a gas furnace unit and a heat pump unit and realize air conditioning of a target space. For example, JP-A No. S64-54160 discloses an air conditioning system that is equipped with a gas furnace unit and a heat pump unit and which, when it starts operating, selects the gas furnace unit as a heat source unit if the outside air temperature is less than a predetermined temperature and selects the heat pump unit as a heat source unit if the outside air temperature is equal to or greater than the predetermined temperature.